The fossil record shows that about 100,000 years ago, several species of hominids populated Earth.

Homo sapiens could be found in Africa and the Middle East; Homo erectus, as typified by Java Man and Peking Man, occupied Southeast Asia and China; and Neandertals roamed across Europe.

By about 25,000 years ago, the only hominid species that remained was Homo sapiens. Scientists have conducted a considerable amount of both genetic and archaeological research in an effort to understand how this outcome occurred.

The two primary theories in the human origins debate are the "Out of Africa" theory and the multi-regionalism theory. Each has its own variations, and there are intermediate models, such as one favoring assimilation among the different groups. Credible evidence exists to support each theory.

The multi-regionalism theory, which relies on fossil evidence, holds that after members of Homo erectus first left Africa roughly 1.7 million years ago, they settled in different regions of the world and evolved separately but concurrently into Homo sapiens. Despite the vast distances, there was enough gene exchange between groups that an entirely new species did not evolve.

The "Out of Africa" theory relies considerably on DNA evidence. This scenario also holds that Homo erectus first left Africa around 1.7 million years ago. Evolution continued, and anatomically modern humans appeared in Africa between 200,000 and 100,000 years ago.

Beginning about 100,000 years ago, these modern humans expanded outside the continent, making their way across Asia and Europe, where they completely replaced the older species, Homo erectus.

Unlike Templeton's assertions, the "Out of Africa" theory does not support the idea of interbreeding between archaic and modern humans.

Guarded Support

Explaining his contention that interbreeding occurred, Templeton said humans "have long shown a pattern of isolation by distance," and at any given time there is some degree of genetic difference between human populations.

"However," he added, "genetic interconnections have long existed among human populations, and this was accentuated by the latest 'Out of Africa' expansion, not eliminated, as under the replacement model."

Templeton's view is "a kind of compromise," said Brooks. "Africa was still the major source of all modern humans, but there was a limited amount of interbreeding with other populations already living in Eurasia," she said.

Fred Smith, a paleoanthropologist at Northern Illinois University who proposed the assimilation model of human evolution, said Templeton's data support the idea that modern humans evolved in Africa, spread to other continents, and interbred with archaic populations.

"I argued for the assimilation model based on morphologywhat could be seen in the fossil record, rather than on genetic evidence. But I'm in agreement with what Templeton has found," Smith said.

Cann, in an accompanying article in Nature, said Templeton's attempt to view the data from a global perspective is over-ambitious given problems with genetic studies of small-scale modern populations.

"I want to see [his methodology and analysis] validated in an area of the world where a variety of scientists from different disciplines think they understand how humans spread and when," she said.

Examples of human migration that might help demonstrate the validity of Templeton's analysis and its limitations, she suggested, include the relatively recent expansion to Polynesia, the spread of farmers from Turkey into Northern Europe, and the migration of Vikings to Iceland.

"We need lots of different tools to study human evolution," Cann pointed out. "Scientists get into trouble when they expect one tool will do everything. Sometimes you need a hammer to attach things, sometimes a screwdriver, and sometimes Velcro works as well!

"Better to keep exploring these different methods with an open mind," she added, "since there are things only fossils can tell you, and things only genetics can reveal."